Monitoring of Hydraulic Fracturing Operations

Abstract

There are disclosed methods and apparatus for monitoring hydraulic fracturing operations, using a distributed optical fibre sensor to detect relevant acoustic signatures, such as acoustic signatures of cement washout and of events involving a valve drive component.

1 . A method of monitoring a hydraulic fracturing operation comprising using a distributed optical fibre sensor to detect an acoustic signature of washout of cement surrounding a casing of a wellbore. 
     
     
         2 . The method of  claim 1  further comprising detecting a spatial extent of said washout from a spatial distribution said acoustic signature. 
     
     
         3 . The method of  claim 1  comprising:
 using the distributed optical fibre sensor to detect an acoustic signal from the wellbore; and 
 identifying the acoustic signature of washout of cement from the acoustic signal. 
 
     
     
         4 . The method of  claim 3  wherein identifying the acoustic signature comprises identifying a central region proximal to an egress point in the casing, and identifying one or more branch regions each of which moves away from the egress point over time in association with related cement washout activity progressing along the wellbore. 
     
     
         5 . The method of  claim 4  wherein identifying the acoustic signature comprises identifying two said branch regions which simultaneously move away from the egress point in opposite directions along the wellbore. 
     
     
         6 . The method of  claim 4  comprising recognizing the central region and the one or more branch regions initiating together at a time of opening of a valve to permit fracture fluid to pass through an egress point in the casing. 
     
     
         7 . The method of  claim 6  further comprising identifying from the acoustic signal a pressure wave front propagating rapidly in both directions along the wellbore, and associating the origin of the wave front with the opening of the valve. 
     
     
         8 . The method of  claim 4  further comprising measuring a spatial extent of said washout from spatial extent of the one or more branch regions. 
     
     
         9 . The method of  claim 1  wherein identifying the acoustic signature comprises identifying an acoustic frequency peak in the acoustic signal detected by the distributed optical fibre sensor. 
     
     
         10 . The method of  claim 9  comprising recognising the acoustic frequency peak as located in a said branch region of the acoustic signature. 
     
     
         11 . The method of  claim 9  wherein the acoustic frequency peak has a height of at least double the associated background acoustic signal. 
     
     
         12 . The method of  claim 9  wherein the acoustic frequency peak has a full width at half maximum (FWHM) of less than 100 Hz. 
     
     
         13 . The method of  claim 9  wherein the apex of the acoustic frequency peak lies between 40 Hz and 300 Hz. 
     
     
         14 . The method of  claim 9  comprising determining extent of the cement washout by determining a spatial position of the acoustic frequency peak 
     
     
         15 . The method of  claim 9  comprising determining packoff of cement washout by determining diminishment of the acoustic frequency peak. 
     
     
         16 . The method of  claim 1  further comprising automatically generating an alarm signal or an alert when an acoustic signature of cement washout is identified. 
     
     
         17 . Apparatus for monitoring a hydraulic fracturing operation comprising:
 a distributed optical fibre sensor comprising a sensor optical fibre disposed along a wellbore; and   a washout detector arranged to receive an acoustic signal from the distributed optical fibre sensor and to detect from the acoustic signal an acoustic signature of washout of cement surrounding a casing of a wellbore.   
     
     
         18 . The apparatus of  claim 17  wherein the washout detector is arranged to detect in the acoustic signal a central region proximal to an egress point in the casing, and one or more branch regions each of which moves away from the egress point over time in association with related cement washout activity progressing along the wellbore. 
     
     
         19 . The apparatus of  claim 17  wherein the washout detector is arranged to detect an acoustic frequency peak in the acoustic signal and to use the acoustic frequency peak in detecting the acoustic signature of washout. 
     
     
         20 . A method of monitoring a hydraulic fracturing operation in a wellbore, the hydraulic fracturing operation including delivering a valve drive component along the wellbore to a valve arranged to permit fracturing fluid to egress from the wellbore, comprising using a distributed optical fibre sensor having one or more sensing fibres disposed along the wellbore to detect an acoustic signal from the wellbore, and identifying from the acoustic signal an acoustic signature of the valve drive component. 
     
     
         21 . The method of  claim 20  wherein the acoustic signature is an acoustic signature of the valve drive component passing along the wellbore. 
     
     
         22 . The method of  claim 21  further comprising deriving a track of the valve drive component from the acoustic signature. 
     
     
         23 . A method of monitoring a hydraulic fracturing operation in a wellbore as set out in  claim 20 , the wellbore being within a rock formation, the hydraulic fracturing operation including delivering a valve drive component along the wellbore to a valve located in the wellbore within the rock formation, the method comprising:
 identifying from the acoustic signal an acoustic signature of the valve drive component engaging with the valve.   
     
     
         24 . The method of  claim 23  wherein the valve is a sliding sleeve valve and the valve drive component is a ball. 
     
     
         25 . The method of  claim 24  wherein the acoustic signature of the valve drive component engaging with the valve is an acoustic signature of the ball becoming seated in the sliding sleeve valve. 
     
     
         26 . The method of  claim 23  further comprising identifying from the acoustic signal an acoustic signature of fracture events in the rock formation resulting from the valve drive component engaging with the valve and consequent successful operation of the valve. 
     
     
         27 . The method of  claim 23  further comprising identifying from the acoustic signal a lack of acoustic signature fracture events in the rock formation resulting from a failure of the valve to operate successfully. 
     
     
         28 . The method of  claim 20  wherein the wellbore is within a rock formation, and identifying from the acoustic signal an acoustic signature of the valve drive component comprises detecting an acoustic signature of a failure of prior engagement of the valve drive component with the valve. 
     
     
         29 . The method of  claim 28  further comprising detecting an acoustic signature of fracture events in rock formations downstream of the valve subsequent to detecting an acoustic signature of a failure of prior engagement of the valve drive component with the valve, and determining whether the failure was due to penetration of the valve drive component through the valve or due to disintegration of the valve drive component dependent upon a length of delay between the two acoustic signatures. 
     
     
         30 . The method of  claim 20  wherein properties of an event involving the valve drive component are calculated from aspects of the acoustic signature propagating away from the event along the wellbore. 
     
     
         31 . The method of  claim 20  wherein the acoustic signature of the valve drive component represents one or two wave fronts triggered at the same time by an event involving the valve drive component, the wave fronts propagating in one or both directions along the wellbore. 
     
     
         32 . The method of  claim 31  wherein the method comprises determining an origin of the two wave fronts and identifying the position and/or time of the event as the origin of the wave fronts. 
     
     
         33 . Apparatus for monitoring a hydraulic fracturing operation comprising:
 a distributed optical fibre sensor comprising a sensor optical fibre disposed along a wellbore; and   a valve drive component detector arranged to receive an acoustic signal from the distributed optical fibre sensor and to detect from the acoustic signal an acoustic signature of the valve drive component.   
     
     
         34 . The apparatus of  claim 33  wherein the valve drive component detector is arranged to generate a position or a track of the valve drive component. 
     
     
         35 . The apparatus of  claim 33  wherein the valve drive component detector is arranged to recognise an impact of the valve drive component at a valve. 
     
     
         36 . The apparatus of  claim 33  wherein the valve drive component detector is arranged to recognise, from the acoustic signal, failure of a valve to open after recognised impact of the valve drive component at a valve. 
     
     
         37 . The apparatus of  claim 33  wherein the valve drive component detector is arranged to recognise, from the acoustic signal, failure of a valve drive component. 
     
     
         38 . The apparatus of  claim 37  wherein the recognised failure is identified by the valve drive component as a failure by extrusion of the valve drive component through the valve. 
     
     
         39 . The apparatus of  claim 37  wherein the recognised failure is identified by the valve drive component as a failure by disintegration of the valve drive component. 
     
     
         40 . The apparatus of  claim 33  wherein the valve drive component detector is arranged to recognise from the acoustic signature two wave fronts triggered at the same time by an event involving the valve drive component, the wave fronts propagating in both directions along the wellbore, and to identifying the position and/or time of the event as the origin of the wave fronts. 
     
     
         41 . (canceled) 
     
     
         42 . (canceled) 
     
     
         43 . A method of monitoring a hydraulic fracturing operation in a wellbore within a rock formation, the method comprising:
 using a distributed optical fibre sensor having one or more sensing fibres disposed along the wellbore to detect a seismic signature of a fracture event in the rock formation, the fracture event resulting from the hydraulic fracturing operation.   
     
     
         44 . The method of  claim 43  further comprising detecting one or more properties of the fracture event from the seismic signature. 
     
     
         45 . The method of  claim 44  wherein one or more of the properties are detected from the spatial development over time of the seismic signature as detected at the one or more sensing fibres. 
     
     
         46 . The method of  claim 44  wherein one or more of the properties are detected from the spatial propagation over time of the seismic signature along the one or more sensing fibres. 
     
     
         47 . The method of  claim 44  wherein the one or more properties include one or more of: a location of the fracture event; a position along the one or more sensing fibres of the fracture event; a (for example radial) distance from the one or more sensing fibres of the fracture event; and a magnitude of the fracture event. 
     
     
         48 . The method of  claim 43  comprising detecting a distance of the fracture event from the one or more sensing fibres using a shape of the seismic signature in a plane having dimensions of time and position along the one or more sensing fibres. 
     
     
         49 . The method of  claim 48  wherein the distance of the fracture event from the one or more sensing fibres is detected from a curve shape of the seismic signature in a plane having dimensions of time and position along the one or more sensing fibres. 
     
     
         50 . The method of  claim 43  comprising detecting a position along the one or more sensing fibres of the fracture event from the position of an apex of a curved shape of the seismic signature in a plane having dimensions of time and position along the one or more sensing fibres. 
     
     
         51 . The method of  claim 43  comprising using the distributed optical fibre sensor to detect seismic signatures of a plurality of such fracture events in the rock formation, and monitoring the hydraulic fracturing operation using the detected seismic signatures. 
     
     
         52 . The method of  claim 51  comprising monitoring a distribution of the fracture events along a fracture zone. 
     
     
         53 . The method of  claim 52  further comprising detecting or measuring pack off from the distribution of fracture events along a fracture zone. 
     
     
         54 . The method of  claim 53  wherein pack off in a particular region of the fracture zone is detected or measured from a diminishment over time in detected fracture events in that region. 
     
     
         55 . The method of  claim 51  further comprising:
 identifying a suboptimal fracture distribution along a fracture zone; 
 including a diverter in the fracture fluid injected into the rock formation for hydraulic fracturing of the fracture zone; and 
 monitoring the effectiveness of the diverter in mitigating the suboptimal fracture distribution using the detected seismic signatures. 
 
     
     
         56 . The method of  claim 55  wherein the suboptimal fracture distribution is also identified using the detected seismic signatures. 
     
     
         57 . (canceled)